One type of prior art stellar navigational system incorporates a tracking sub-system which, together with a flight computer, computes the position of a selected star relative to the vehicle in which the sub-system is mounted, searches out the star, tracks the star accurately, and determines the terrestial position of the vehicle. The sub-system includes a telescope which during operation of the sub-system is locked onto the star; and the sub-system also includes a vidicon or solid state stellar sensor. An image of the star is focused onto the surface of the stellar sensor by the optical system of the telescope. By using a closed servo loop, the corrections from the tracking sub-system can be used to correct the values of input latitude and longitude, so that latitude and longitude counters can be up-dated as long as the tracking sub-system is locked onto the selected star.
The tracking sub-system is gyro stabilized, such stabilization being achieved by mounting the sub-system on a stable platform in an inertial measuring unit. The inertial measuring unit is a self-contained system which can automatically maintain angular reference directions in inertial space. The inertial measuring unit includes a platform supported, for example, on three gimbals. The tracking sub-system is mounted on the platform, as are, for example, three single-axis gyros designated the X-gyro, the Y-gyro and the Z-gyro. Any drift of the platform from the attitude prescribed by the gyros causes one or more of the gyros to generate signals, each of which is applied in a corresponding servo loop to a corresponding torquer motor which, in turn, applies a correction torque to the corresponding gimbal to return the platform to its stabilized position. Accelerometers are also mounted on the platform to measure the acceleration of the vehicle along each of the three coordinate axes.
Another type of stellar navigational system includes a position-sensing sub-system which is sighted on a selected star, and which causes an error signal to be generated if the actual relative position of the star differs from the position it would have if the vehicle were on course. The error signal is used to make the necessary corrections on the navigation system.
The autocollimator assembly of the present invention may be used in either the tracking or position-sensing sub-systems described above. The assembly of the invention, in conjunction with a number of position-indexing mirrors mounted at selected angular positions on the internal surface of the spherical case of the inertial measuring unit of the stellar navigation system in which the autocollimator assembly is installed, measures sensor misalignments and scale factor; accelerometer input axes bias, scale factor and misalignments; gyro drift; and the like, with the accelerometers, gyros and sensor actually installed in the space vehicle. The autocollimating assembly of the present invention is advantageous in that it permits greater accuracy than can be achieved with the prior art systems which use external equipment and electro-mechanical transfer devices, such as gimbal angle synchros, since the assembly of the invention makes direct measurements on the components and elements of the navigational system. The autocollimating assembly of the invention is also advantageous in that it is capable of making its measurements while the elements and components of the tracking or position-sensing sub-system are actually installed in the vehicle, which allows shorter periodic up-dating of instrument parameters to minimize errors caused by long term shifts and uncertainties between factory calibrations and installation.
Briefly stated, the invention provides an electro-optical system for accurately indexing the sensor of the tracking or position-sensing sub-system of a stellar navigational system relative to the gravity and earth rate vectors. This is accomplished, as will be described, by using different gimbal orientations, in which the tracking telescope is directed at different ones of the position-indexing mirrors on the internal surface of the case of the inertial measuring unit, and making measurements for each orientation. The different positions are selected so that each accelerometer and each gyro experiences +1G, 0G, -1G; and either along or perpendicular to the earth rate vector, this being important for gyro calibration.